Most of the conventional magnetic recording media are of the coated type which is produced by dispersing particles of magnetic oxides such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 and CrO.sub.2 or ferromagnetic alloys such as Co-Fe-Cr in an organic binder such as a vinyl chloride/vinyl acetate copolymer, styrene/butadiene copolymer, epoxy resin, or polyurethane resin, applying the resulting coating solution to a non-magnetic base, and drying the coating. However, due to a recent increase in the demand for higher density recording, researchers' attention has been drawn to binder-less magnetic recording media that are free from an organic binder and which use as a magnetic recording layer a thin ferromagnetic metal film that is formed by vapor deposition such as vacuum deposition, sputtering, ion plating or chemical vapor deposition (CVD), or plating such as electroplating or electroless plating. Efforts are being made to use such media on a commercial basis.
Most conventional magnetic recording media of the coated type use a magnetic material that primarily consists of a metal oxide having a smaller saturation magnetization than ferromagnetic metals. Therefore, the magnetic recording layer which must be as thin as possible for higher density recording cannot be made thinner than a certain value beyond which a drop in reproduction output occurs. As a further disadvantage, the process of manufacturing this type of recording medium involves complex steps, as well as a large separate apparatus for solvent recovery or pollution control. One merit of the binder-less magnetic recording medium is that a very thin magnetic recording layer necessary for high-density recording can be formed by using a ferromagnetic metal having a greater saturation magnetization than the magnetic oxides without using a non-magnetic material such as organic binder. Another merit is that this type of recording medium can be made more simply than magnetic recording medium of the coated type.
Studies have been made on the production of the binder-less magnetic recording medium by vapor desposition or ion plating because unlike solution plating treatments, these techniques require no effluent treatment and instead involve simple procedures and can form a film of a desired thickness by using a particular condensation rate (deposition rate). Several methods of vapor deposition are known for producing a magnetic film having a coercive force and squareness ratio desired for a good magnetic recording medium, and three of them are listed below:
(1) Method of controlling the degree of vacuum or vapor deposition rate such as disclosed in A. V. Davies, et al., IEEE Trans. Magnetics, Vol. MAG-1, No. 4 (1965), p. 344; U.S. Pat. No. 3,787,237;
(2) Oblique deposition wherein beams of the vapor of a ferromagnetic metal are directed to a substrate at an angle such as disclosed in W. J. Schule, J. Appl. Phys., Vol. 35 (1964), p. 2558; U.S. Pat. Nos. 3,343,632 and 3,342,633; and
(3) Method of condensing the vapor of a ferromagnetic metal on a Cu-base substrate under heating such as disclosed in U.S. Pat. No. 4,226,681.
However, these methods require additional refinements since they are not able to form a magnetic recording medium having the desired magnetic properties. They involve complex procedures of vapor deposition, or they can only be applied to limited types of substrates.
The ion plating process can provide a magnetic film having great coercive force if the gas used is introduced at high pressure. However, the resulting film does not have sufficient adhesion with the substrate. If the gas is introduced at low pressure, the film obtained has very good adhesion with the substrate but its magnetic properties are so poor that a practical magnetic recording medium cannot be obtained.
The binder-less magnetic recording medium using a thin ferromagnetic film produces a greater reproduction output than the conventional magnetic recording medium of coated type, but it also has relatively high noise, and the resulting S/N ratio (signal to noise ratio) is not completely satisfactory. For this reason, noise reduction is one of the objectives in the current development of a commercial binder-less magnetic recording medium.